While I am not fund of deaths in stories, I hate when they are done in pointless manners or for pointless reasons. Take Naruto. Kushina, Minato, Jiraya all died for valid good reasons. However, Sarutobi for example died for a stupid reason when we came to finding out he was a master ninja of incredible power that lost against meh power. His death advanced the plot, yes, but the plot could advance with him alive in the same manner. However, Naruto wouldn't be who he was if his parents and mentor were alive.
In this case, even if Juvia dies, I don't see that big of an effect in Gray. His determination to kill END and the enemies is the same. And I doubt he'll "power up" any different than he has all other power up moments before.
I think Makarov could have died to empower the guild as he had already shown his true power in the early arcs and is now subdued to showcase the other guild members powers for example. Is useless now and isn't needed anymore. Might as well kill him off for some good. But Juvia? She never even managed to grow and show her real power.
